1. Field of the Invention
This invention relates to nondestructive testing of pavements and in particular to testing of pavements using rolling wheel deflectometers along with triangulation processes.
2. Description of the Related Art
Load bearing capability for airport runways or highway pavements may deteriorate, over time, due to a number of factors, including changes in the elastic moduli of subpavement layers of earth. In order to determine pavement condition for airport runways or highways, the load bearing capability of the pavement can be periodically tested. In order to measure the load bearing capability of the pavement, it is desirable to utilize technologies that are nondestructive so that the integrity of the pavement layer is maintained. Further, the measurements should desirably be made rapidly, through an automated system, to minimize time and reduce costs.
Different methods have been developed for the non-destructive testing of pavements, with one utilizing a falling weight dropped on the pavement from a stationary platform. Sensors then measure the deflection of the pavement at intervals out from the falling weight. Systems utilizing this method are commonly referred to as falling weight deflectometers (FWD or FWDs).
Other systems utilize a fast moving, heavy wheel load that rolls along the pavement, with sensors being arranged and intervals out from the wheel to measure deflection. Systems utilizing this approach are commonly referred to as rolling weight deflectometers (RWD or RWDs). A device of this type is disclosed in U.S. Pat. No. 4,571,695. In essence, a load is placed on a wheel that rolls across the pavement and the depth of a deflection basin created by the loaded wheel is measured using precision laser sensors mounted on a horizontal member that tracks with the wheel. Such deflection measurements provide insight into the load bearing capability of the pavement. However, pavement deflections are usually very small, typically 0.010 to 0.040 inch for a 20,000 pound applied load. Therefore, not only are extremely sensitive sensors required to measure the deflection, but the sensors should have a stable reference plane.
The deflection of the pavement surface under a fast moving, heavy wheel load may be measured through triangulation. A simple algorithm for a system using four sensors was developed by Professor Milton Harr of Purdue University (See Harr, M., and N. Ng-A-Qui, 1977, “Noncontact, Nondestructive Determination of Pavement Deflection Under Moving Loads,” FAA-RD-77-127, U.S. Department of Transportation, Washington D.C).
The Harr algorithm was published 30 years ago and several systems for road testing are based on this approach. The method is illustrated in FIG. 1. (See Briggs, R. C., Johnson, R. F., Stubstad, R. N., and Pierce, L., “A Comparison of the Rolling Weight Deflectometer with the Falling Weight Deflectometer,” Nondestructive Testing of Pavements and Backcalculation of Moduli: Third Volume, ASTM STP 1375, S. D. Tayabji and E. O. Lukanen, Eds., American Society for Testing and Materials, West Conshohocken, PA, 1999.)
Using the approach shown in FIG. 1, the sensors are placed at equal distances apart and two sets of measurements are carried out, at points with spacing equal to the distance between the sensors. From the first set of measurement the distance, h, from a reference line, through points P1 and P2, to the pavement surface at point P3 is calculated from:h=A−2×B+C where A, B and C are the measured distances from the beam to the pavement surface at points P1, P2 and P3, respectively, during the first set of measurements.
From the second set of measurements the same distance may be calculated from:h′=B′−2×C′+D′where B′, C′ and D′ are the measured distances from the beam to the pavement surface at points P1, P2 and P3, respectively, during the second set of measurements.
If there is no deflection under the load of the wheel, h′ and h will be identical, so the difference between h′ and h is the deflection of the pavement caused by the wheel load. The combination the equations above is commonly known as “Harr's algorithm”. Harr's algorithm is typically correct if it is assumed that only the pavement at the position of one sensor is influenced by the deflection from the wheel, whereas the remaining sensors are assumed to be outside of the deflection caused by the wheel load, i.e. the deflection basin. That is, the wheel load is assumed not to spread beyond the immediate area of the wheel load to cause any changes at locations other than the one sensor at the wheel load. This assumption is not always correct, and may only be correct for pavements with very shallow depth to bedrock.